AngleViaPoint: placement of tangency icon in 3dview

Implemented placement of tangency-via-point icon in 3d view. Also
affected is the placement of point-on-object icon (since it is very
similar code, it is now shared with tangency-via-point)
AngleViaPoint: placement and moving of angle datum

+ small changes/fixes in point-on-object/tangent-via-point icon
placement